In HPLC, a sample to be investigated has to be fed into a high-pressure liquid stream, wherein the latter should only be interrupted for as short a period of time as possible. For this purpose, high-pressure switching valves in the form of high-pressure injection valves are used which allow virtually interruption-free switching of the liquid stream. Such a structure is described for example in U.S. Pat. No. 3,530,721, the original application for which dates back to as early as 1965.
The further development of such an injection valve is mentioned for example in U.S. Pat. No. 4,242,909. The basic principle of the valve shown therein has become largely established in the meantime in HPLC. Since the present invention is based on this valve type, the principle is explained in more detail in the following text.
FIG. 1 shows a schematic illustration of such a high-pressure valve according to the prior art. It consists of a stator 112 and a rotor 106. The stator 112 has a total of six input and output ports 118. Via these ports, the injection valve can be connected to the other functional elements of the HPLC system via capillary connections. The port connections and high-pressure screw connections required for this purpose are not illustrated in FIG. 1 for the sake of clarity. Within the valve, the ports are in the form of ducts, for example in the form of holes, which lead to the stator end face 114 of the stator 1. In contrast to the simplified illustration in the drawings, in the case of valves produced in practice, the pitch circle diameter on the side of the port connections is usually larger than on the stator end face 114. The rotor has a number of arcuate grooves 108 which are oriented precisely with the holes in the input and output ports or the port opening cross sections thereof in the stator end face 114. This is indicated in FIG. 1 by way of dotted lines. In order to provide a clearer illustration, the rotor 106 is shown at a distance from the stator 112 in FIG. 1. In the assembled state of the valve, this distance is zero, and therefore the surface 110 of the rotor 106 lies directly against the stator end face 114 of the stator 112, as is shown in FIG. 2.
At this point, it should be mentioned that the valve according to FIG. 1 can of course also be used for other purposes than for the purpose of injection.
FIG. 2 shows a schematic illustration of an operationally assembled valve according to the prior art. The rotor 106 is pressed against the stator 112 with a pressure force which is indicated by the arrow F, such that a common interface 110 is formed between the rotor 106 and the stator 112, the two parts sealing against one another at said interface 110. The pressure force F is in this case measured such that the arrangement is still sealed even at the highest pressures to be expected.
In the first switching position, shown in FIG. 1 and FIG. 2, of the valve, the grooves 108 are oriented with respect to the port opening cross sections of the input and output ports 118 such that they produce three connections between in each case two adjacent input and output ports. On account of the sealing action at the interface or contact face between the rotor 106 and the stator 112, liquid supplied to a port 118 can thus emerge only at the relevant adjacent port 118.
In order to switch the valve into a second switching position, the rotor 106 can be rotated through 60° with respect to the stator 112 such that the grooves now connect together in each case those ports which previously had no connection. The direction of rotation is indicated in FIG. 1 by an arrow on the rotor. However, the direction of rotation can also to be selected to be in the opposite direction.
Switching is usually executed by a motor-powered drive which can rotate the rotor 106 with respect to the stator 112. For the sake of clarity, the drive has been omitted in the drawings. In principle, switching of the valve can also take place manually, however.
The advantage of such valves is that they can be used for very high pressures, given a sufficiently high-pressure force F. Furthermore, the holes in the ports 118 can be arranged such that the ends lie on a circle with a very small radius. The grooves then likewise lie on a circle with a very small radius such that the dead volumes of the valve can be kept very small.
In HPLC, a trend toward separation columns having a small particle size has been observed in recent years. Such separation columns allow an improved separation performance and more rapid separation, for which reason the expression fast HPLC is used.
Since the flow resistance increases very greatly as the particle size drops, considerably higher pressures are necessary for fast HPLC. The maximum column pressure that occurs is typically between 100 and 400 bar in conventional HPLC, while 600 to 700 bar are usually necessary in fast HPLC, sometimes even over 1000 bar. A trend toward columns with an even better separation performance is already being observed, said columns requiring even higher pressures of up to about 2000 bar.
In order to be able to operate high-pressure injection valves at such high pressures, the pressure force F (see FIG. 2) has to be increased in a corresponding manner, in order that the valve is sealed. In order that the rotor, which is normally produced from plastics material for cost and technical reasons, can withstand this force, glass- or carbon-fiber-reinforced plastics materials are used according to the prior art. Furthermore, the increased pressure force F results in increased material stress and consequently excessive wear, such that the service life of the valve (number of switching cycles) is unsatisfactory.
This problem can be solved by an appropriate material selection or coating. Thus, a special coating which allows cost-effective production of rotor and stator and at the same time greatly reduces the wear of the materials is described in U.S. Pat. No. 6,453,946.
WO 2009/101695 describes a switching valve in which the stator is provided with a coating made of amorphous carbon (DLC coating) in order to improve stability. The end face or contact surface of the rotor consists of a synthetic resin.
However, it has been shown that although such improved valves behave more favorably, during operation at very high pressures, they nevertheless fail even after a relatively small number of switching cycles.
US 2010/0281959 A1 describes a switching valve suitable for high pressures, wherein the stator and/or rotor surfaces are provided with a DLC layer, wherein an adhesive layer is provided between each particular main body, which can consist of metal. However, when hard material is used in each case for the rotor and the stator, there is the risk that increased wear will occur on account of irregular surface pressure at the contact face, since hard main bodies scarcely deform at the contact face.
Therefore, it is the object of the invention to create a high-pressure switching valve for high-performance liquid chromatography, which has improved wear resistance and stability and can nevertheless be produced in an easy and cost-effective manner.